Incoming device as Disconnector Switch / Isolator

[NickParker]

See the picture below, Can the incoming device be a disconnector switch or a circuit breaker?
I am looking for both IEC and NEC codes that permit the use of disconnector switches as incomers.

 

[waross]

The device must be rated for the application.
For instance, if the example is ruled a service entrance by the AHJ then the device must be "Service Entrance"rated.

You may have an instance where the upper device on the transformer secondary must be "Service Entrance" rated, but the lower device need not be.
If the device may be opened under load, then the device, whatever is used, must have an adequate ASSC rating, as well as adequate voltage and current ratings.

 

[jghrist]

| don't think it is either an IEC or ANSI symbol. Looks more like a horn gap switch symbol, but that would not provide the protection needed.

 

[crshears]

Why are the reply dates on this post getting earlier and earlier in time?

 

[crshears]

And I'm posting this on 3 December 2024, not 23 March 2013 . . .

 

[jghrist]

I think the Mar 23, 2013 date is when you set up your Eng-tips account.

 

[stevenal]

Shows as Mar 22 on my screen.

The posts all show time of day in my time zone, yesterday and today in sequence.

 

[waross]

"Look up, way up"
The time elapsed since a reply was posted is given on the left end of the blue line immediately above the reply.
I am seeing this:
"Today at 10:07 AM"
for the post above this reply.

 

[Padlock01]

The switch disconnector on the MCC will not discriminate between the outgoing MCCBs if the outgoing MCCB's protection is not set properly
then you will trip the MCCB / ACB feeder on the Main LV switchboard.

 

[crshears]

I think the Mar 23, 2013 date is when you set up your Eng-tips account.

Ah! so that's not the date of the reply; sorry for the confusion.

 

[Gr8blu]

The main function of a contactor is to repeatedly energize and de-energize a connected load. This functionality is typically enabled using distributed control systems, using automated programmable logic controllers (PLC) or in combination with the more traditional start-stop control pushbutton station, along with protective devices such as fuses and protection relays. On the other hand, a circuit breaker’s main function is circuit protection. Its primary role is to protect an electrical installation, or portions of the installation from fault conditions and to provide circuit level isolation.

The misuse of IEEE Device 52 occurs when a motor bus does not distinguish which motors are controlled with contactors and which motors are controlled with circuit breakers. The use of a motor control contactor (MCC) brings several significant benefits in comparison to a circuit breaker, including smaller space requirements, higher duty cycles and longer life.

However, contactor-based MCCs cannot perform the same functionality of the circuit breaker and vice versa. For example, more care must be used when applying fused contactors on solidly grounded neutral systems because these devices are not rated or designed to interrupt high fault currents. The interrupters used in a contactor can typically only switch between 5 kA to 14 kA. In contrast, the interrupters used in circuit breakers can interrupt much higher currents - typically in the range of 40kA to 63kA.

As a result, overcurrent, ground-fault and differential protective elements/functions (50/51, 50N/51N, and 87 respectively) should not be enabled in a way that could open the contactor at interrupting ratings higher than the contactor’s unfused rating. These faults could be up to the available fault current in some scenarios. MCC manufacturers will perform short-circuit tests on their combination controller (fuses and MV contactor) to confirm safe performance on the system short-circuit levels within its equipment’s rating.

For a good discussion of how this all works, refer to IEEE 3004.8 (which is an updated version of a portion of the IEEE "color books" related to industrial and commercial power system protection).

 

[AlBerton]

See the picture below, Can the incoming device be a disconnector switch or a circuit breaker?
I am looking for both IEC and NEC codes that permit the use of disconnector switches as incomers.

View attachment 1817

Yes it can be a disconnector switch, but it shall be backupped from the upstream CB, meaning that in case of short circuit the upstream CB shall let pass an amount of energy below the maximum acceptable for the switch disconnector (without being damaged). Every manufacturer has its own coordination tables where you will find specified the downstream SD, the upstream CB and up to which short circuit current the back up is guaranteed. This of course implies that CB and SD shall be of the same brand, as these tests are laboratory made and every manufacturer performs them with his equipment only.
Finally, the MCC feeders shall be coordinated with the upstream CB, i.e. they shall open faster than the main CB for any fault on the protected lines.

 

[lmarcantonio]

| don't think it is either an IEC or ANSI symbol. Looks more like a horn gap switch symbol, but that would not provide the protection needed.

That's an IEC symbol for a breaker (EN617-7 07-13-05) which is actually *not* a disconnector device; the cross is the power switch designator, a proper disconnector has a "T" head. At 0.4kV usually breakers also have disconnecting function (i.e. disconnection under load) but that's not a rule. Since 'disconnecting' is a property depending on contact gap a breaker can disconnect only if it is designed to do so (also important the breaking capacity if under load).

 

[JezNZ]

Since this is LV. Use a load-break isolator, not a disconnector (off-load). There's no real purpose to put circuit breakers on both the supply side and at the incomer of a board unless this something like a rising main configuration, where that incomer will also serve other boards.